The present invention relates generally to optical communications, and more particularly, to optical spectral shaping for nonlinearity enhancement in high speed digital coherent optical transmission.
Coherent optical communication has triggered the intensive research in improving system capacity, transmission distance and spectral efficiency by using high-order modulation formats and advanced digital signal processing (DSP) techniques. Among the various modulation formats, dual-polarization (DP) quadrature phase shift-keying (QPSK) is one of the popular candidates for upgrading system capacity at 100 Gb/s per channel with trans-oceanic transmission reach [1]. Instead of laying new dispersion uncompensated fiber links (which have the optimal dispersion map for digital coherent transmission systems), it is highly desirable to upgrade the widely deployed dispersion managed fiber (DMF) links to 100 Gb/s and beyond to reduce the system cost and increase the network data capacity. However, one of the challenges in digital coherent transmission over DMF links is caused by the its chromatic dispersion (CD) map, which was designed to have full CD compensation by using dispersion-compensated fiber (DCF) for analog receivers. This type of dispersion map would bring two critical issues when upgrading such system to 100G using coherent receivers: the small effective area of DCF (˜25-40 μm2) brings strong self-phase modulation (SPM) effects; the dispersion map does not provide large walking-off between neighboring WDM channels, thus enhancing the cross-phase modulation (XPM) impact. As a result, the 100G upgrade over legacy DMF fiber is a very challenging yet beneficial project for both system providers and network carriers.
With the aid of coherent receivers, enhanced DSP algorithms have been proposed to compensate for linear distortions and even fiber nonlinearity, such as SPM and XPM. Recent demonstration has shown 1.7 dB Q-factor improvement using SPM compensation and maximum likelihood sequence estimation. Compared with new advanced DSP algorithms, which generally require high investment for developing new ASIC chips, innovative optics solutions can be more attractive. In this paper, we propose to use low cost multi-channel optical spectral shaping device for enhancement of the fiber nonlinearity tolerance of 100G DP-QPSK transmission over legacy DMF fibers.
Optical fiber nonlinearity is one of the major limiting factors for high-speed optical communications. In the past, various measures have been proposed to improve the optical transmission system tolerance to fiber nonlinearity. As for digital coherent optical communication systems, some of the prior research efforts include the following outputs:
Since digital coherent receivers can have high tolerance to fiber chromatic dispersion, one approach is to adopt dispersion uncompensated link for high-speed optical transmission systems, and all the residual dispersion is compensated by the electronic digital signal processing (DSP) units at the receiver end. For example, an optical link consists of uncompensated standard single mode fiber (SMF-28) with dispersion at ˜18 ps/nm/km. However, in transoceanic optical cable communications, very large dispersion can be accumulated and go beyond the dispersion compensation capabilities of the digital coherent receiver. Therefore, dispersion managed fiber links, which can manage the total residual dispersion, are more practical and popular for undersea optical cable communication systems. As pointed in session A1, dispersion managed fiber links generally incur large fiber nonlinearity to the optical signals.
In a digital coherent optical communication system, fiber nonlinearity has been shown to be compensated through high-speed DSP processing. One fiber nonlinearity compensation algorithm is known as “back propagation” algorithm, in which the received signal is back-propagated thorough the transmission line using DSP processing. Due to the lack of the analog solution to the Nonlinear Schrodinger Equation describing the optical signal propagation in optical fiber, the back propagation algorithms virtually solve the Nonlinear Schrodinger Equation wither reverse signs numerically. The existing nonlinearity compensation algorithms are generally demanding on computation resource and requires high investment for building new ASIC chips.
Accordingly there is a need for improved optical spectral shaping for nonlinearity enhancement in high speed digital coherent optical transmission